Enhanced ferromagnetism in single crystalline Co-doped ZnO thin films by Al codoping

High-quality (Co, Al)-codoped ZnO single crystalline films have been grown on a-plane sapphire substrates using molecular-beam epitaxy. Al codoping yields the Co-doped ZnO thin films to exhibit metallic conducting behavior with high fee carrier concentration. X-ray absorption studies confirm that nearly all Co ions are in divalent state and actually substituted into the ZnO lattice without formation of any detectable secondary phase. Compared with weak ferromagnetism (0.32 μB/Co2+) in the Co-doped ZnO single crystalline film, the film with additional Al codoping was found to have much stronger ferromagnetism (0.83 μB/Co2+) at room temperature. The magnetic anisotropy was also observed in the (Co, Al)-codoped ZnO thin film, indicating the ferromagnetism is intrinsic. Our experimental observations suggest that additional electron doping is effective to strengthen the ferromagnetism in Co-doped ZnO films, which is well consistent with the recent theoretical description of carrier-mediated magnetism in Co-doped ZnO.